A semiconductor integrated circuit (IC) is required to be resistant to surge voltage and surge current caused by electrostatic discharge (ESD) and loaded on the input/output pad of the IC. In order to meet this requirement, an ESD protection circuit is connected to the input/output pad of IC in general. Such an ESD protection circuit is required to have a good discharge ability and low trigger voltage as semiconductor integrated circuits are miniaturized. An example of an ESD protection circuit that meets these requirements is disclosed in Patent-Document 1.
FIG. 4 is a cross-sectional view showing the structure of the main part of a conventional ESD protection circuit. On a P-type semiconductor substrate 100, an N-type well 101 and a P-type well 102, adjacent to the N-type well 101, are formed. Furthermore, on the surface of the N-type well 101, a high concentration N-type region (N+ region) 103 and a high concentration P-type region (P+ region) 104 are formed, and a high concentration N-type region 105 is formed on the surface of the P-type well 102. Element isolation films 106a, 106b, 106c, 106d are isolation films to electrically isolate elements from each other, particularly the element isolation film 106b isolates the high concentration N-type region 103 from the high concentration P-type region 104, and the element isolation film 106c isolates the high concentration N-type region 105 from the high concentration P-type region 104. The high concentration N-type region 103 is connected to a trigger device not shown in the drawing, the high concentration P-type region 104 is connected to an I/O pad also not shown in the drawing, and the high concentration N-type region 105 is grounded.
FIG. 5 is a drawing showing the ESD protection circuit shown in FIG. 4 as an equivalent circuit diagram. Further, FIG. 6 is a layout diagram looking at the ESD protection circuit in FIG. 4 from above. An xy section in FIG. 6 corresponds to FIG. 4. In FIGS. 5 and 6, a PNP transistor Tr1 is comprised of the high concentration P-type region 104, the N-type well 101, and P-type well 102, and an NPN transistor Tr2 is comprised of the N-type well 101, the P-type well 102, and the high concentration N-type region 105. The transistors Tr1 and Tr2 constitute a silicon controlled rectifier (SCR), the high concentration P-type region 104 becomes an anode electrode of the SCR, the high concentration N-type region 105 becomes a cathode electrode of the SCR, and the high concentration N-type region 103 formed on the N-type well 101 becomes a trigger electrode of the SCR.
In this ESD protection circuit, a trigger current Itrig flows from the I/O pad to the trigger device when a voltage higher than a predetermined value is applied to the trigger device. This trigger current acts as a base current of the PNP transistor Tr1, the PNP transistor Tr1 is turned on as a result of the base current flowing, and a collector current flows. When a voltage drop caused by the collector current of the PNP transistor Tr1 occurs in a resistance Rpwell inside the P-type well 102, the base and emitter junction of the NPN transistor Tr2 is forward-biased and the NPN transistor Tr2, too, is turned on. The operation of SCR is achieved by turning on the both transistors Tr1 and Tr2 as described above.
Meanwhile, a structure wherein two systems of SCR are provided in parallel is described in Patent Document 2. FIG. 7 is a drawing showing the layout of the ESD protection device in Patent Document 2. In the ESD protection device shown in FIG. 7, with an N+ region 123 for an N-well potential fixing electrode as the center, N+ regions 124 for a trigger electrode, P+ regions 125 for an anode electrode, and N+ regions 127 for a cathode electrode are formed in order vertically and symmetrically. Outside of these, P+ regions 131 for a P-well potential fixing electrode are provided. P+ regions 125 for the anode electrode, an N-well 121 (the N+ regions 124 for the trigger electrode), a P-well where the P+ regions 131 for the P-well potential fixing electrode are provided, and the N+ regions 127 for the cathode electrode constitute the SCR structure.
[Patent-Document 1] Japanese Patent Kokai Publication No. JP-P2003-203985A (FIG. 2)
[Patent-Document 2] U.S. Patent Publication US2004/0136127A1 (FIG. 18B)